Current-Mode control of Thermo-Electric cooler

ABSTRACT

According to an exemplary embodiment of the present invention, an apparatus for controlling the temperature of a thermoelectric cooler includes a current sensing circuit which is adapted to control the current through the thermoelectric cooler using proportional, integral, derivative (PID) processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority from U.S. ProvisionalPatent Application Serial No. 60/311,496, filed Aug. 10, 2001, entitled“Current-Mode of Thermo-Electric Cooler.” The disclosure of thisabove-captioned provisional patent application is specificallyincorporated by reference herein for all purposes.

FIELD OF THE INVENTION

[0002] The present invention relates generally to opticalcommunications, and specifically to an apparatus for controlling athermoelectric cooler (TEC) in order to maintain a substantiallyconstant temperature on the surface of the TEC.

BACKGROUND OF THE INVENTION

[0003] Digital optical communications have gained widespread acceptancefor both telecommunications (telecom) and data communications (datacom)applications. Telecommunication systems typically operate oversingle-mode fiber at distances from 10 km to over 100 km, and employlasers emitting light at wavelengths of 1310 nm to 1600 nm. Datacommunication systems typically cover shorter distances of up to a fewkilometers, over multi-mode fiber. Data communication systems can employlaser devices as well, typically having emission wavelengths of 650 nmto 850 nm. As the transmission and reception rates in the telecom anddatacom industries continue to increase, there are ever increasingdemands placed on the various components of the optical communicationsystem.

[0004] One such demand is the continuity of the laser output over timeand ambient temperature. As is known, temperature variations canadversely impact the operation of a semiconductor laser. To this end,the wavelength and power output of a semiconductor laser may beinfluenced by temperature fluctuations. As can be appreciated, inoptical communication systems, (e.g., dense wavelength divisionmultiplexing (DWDM) systems) wavelength variation must be maintainedwithin specified boundaries. Output optical power variations must alsobe minimized in order to maintain certain target operating parameterssuch as extinction ratio and average output power. Therefore, tofacilitate laser operation within close tolerances for both wavelengthand optical power, it is useful to closely control the temperature ofthe active layers of a semiconductor laser.

[0005] One method to control the operating temperature of a laser isthrough the use of a thermo-electric cooler (TEC). A TEC is a devicethat may be controlled to either add or extract heat to/from a laser.However, in conventional uses of the TEC the amount of power used tocool a laser to a desired temperature often exceeds the amount of powerthat is used to drive the laser. This can result in an undesirablecondition known as thermal runaway, since the additional power suppliedto cool the laser may actually increase the temperature of the operatingenvironment of the laser, rather than decreasing it.

[0006] One conventional thermoelectric cooler is a Peltier effectthermoelectric cooler (Peltier TEC). A Peltier TEC may consist of pairsof p-type and n-type materials connected in series and sandwichedbetween two closely spaced ceramic plates. When connected to a DC powersource, current flow through the series of p-n junctions transfers heatfrom one side of the thermo-electric cooler to the other. In a typicalapplication, the “cold” side of the thermoelectric cooler is connectedto the device to be cooled, while the “hot” side is connected to a heatsink to disburse the heat to the outside environment. Of course, bychanging the current direction, the thermo-electric device can operateas a heater.

[0007] Peltier TEC's are often incorporated into laser assemblies tocontrol the temperature of a semiconductor laser over wide ambienttemperature ranges. The temperature of the laser may be monitored with anegative temperature coefficient (NTC) thermistor circuit, whichprovides feedback to the closed-loop temperature control block. Themaximum thermo-electric cooler temperature differential may be achievedwith the application of a maximum voltage, V_(max), which is temperaturedependent, and results in the flow of a maximum current I_(max), whichis substantially temperature independent. Voltages and currents abovethese levels may result in the opposite effect intended, mainlyincrementally heating when cooling is desired. As such, overheating andthermal runaway of the TEC circuit may result if the voltage and/orcurrent applied to the TEC is not bounded or limited in some manner.

[0008] One known technique for controlling a TEC is known asvoltage-mode control. TEC voltage-mode control results in a closed-loopcontrol circuit that has a temperature dependent closed-loop response.This is because the TEC voltage varies as a function of the temperaturedifferential across the TEC, due to a temperature dependent back-EMFthat results from the Peltier effect. The TEC voltage-mode controlresults in a closed-loop circuit, the performance parameters of whichare also subject to a relatively large initial tolerance due tovariation in the semiconductor characteristic resistance. Ultimately,these variations in the relationship of heat transfer as a function ofTEC applied voltage significantly limit the performance of voltage-modecontrol of the TEC.

[0009]FIG. 1 is a schematic diagram of a conventional TEC-drive circuit100 used to implement closed-loop temperature control of a TEC. The TECdrive circuit 100 provides voltage-mode control of a TEC 106. A controlblock 101 consists of a temperature monitor circuit 102, which monitorstemperature feedback; a set-point input 103 for target temperaturecontrol; and a proportional, integral, derivative (PID) control block104 ^(a). The PID control block 104 may be implemented via a firmwarecontrol method driving a digital to analog converter (DAC); or entirelyin analog circuitry (e.g., operational amplifiers). The output of thecontrol block 101 drives a summing op-amp 105 which controls the voltageacross the thermo-electric cooler 106, as a function of the output ofthe control block 101. A reference voltage 107 is summed at op-amp 105to provide an offset to allow positive and negative (bipolar) driving ofthe thermo-electric cooler (TEC) 106 with a unipolar (PID) signal 104. Aunity gain buffer 108 provides the TEC 106 drive current in excess ofapproximately 1 amp to approximately 2 amps, depending upon themanufacturer's specification for I_(max) of the TEC.

[0010] In order to suitably protect the TEC 106 from exceeding themaximum current, I_(max), the current is monitored with a currentmonitoring/protection circuit 112 which includes a sense resistor(R_(sns)) 109, a first transistor (Q_(p)) 110 and a second transistor(Q_(n)) 111. The voltage across the sense resistor 109 turns the firsttransistor 110 or the second transistor 111 on in an over-currentcondition, thus shunting the op amp drive signal, protecting the TECfrom an over-current condition.

[0011] In order for the sense resistor 109 to turn on first transistor110 or second transistor 111 in an over-current condition, the voltagedrop must be equal to the base-emitter junction turn-on voltage(V_(be)). However, this turn-on voltage is a function of temperature. Asthe temperature of the first and second transistors increases, theturn-on voltage is reduced, and the current monitor protection circuit112 can begin shunting the drive signal prematurely. Thereby, the fullrange of the TEC is not realized. Further exacerbating this issue is thefact that the sense resistor 109 is relatively large, and dissipatessignificant power. This can add to the temperature variability of thefirst and second transistors 110 and 111, as well as make the overallTEC drive circuit 100 very inefficient from a power perspective.

[0012] As can be appreciated, using the voltage-mode control techniquedescribed above may prohibit the use of the full cooling range of theTEC, resulting in a reduced operational range for laser temperaturecontrol. Moreover, the TEC may enter self-heating mode(overheating/runaway) before current limiting occurs. Also, therelatively large voltage drop across the sense resistor 109 may causethe current buffer 108 to reach voltage saturation in low voltageapplications. This may also limit the cooling range of the TEC.Additionally, the power rating of the sense resistor 109 dictates alarge package size, which consumes valuable board real-estate.

[0013] Accordingly, what is needed, is a technique for controlling athermo-electric cooler that overcomes at least the drawbacks of theconventional art described above.

SUMMARY OF THE INVENTION

[0014] According to an exemplary embodiment of the present invention, anapparatus for controlling the temperature of a thermo-electric coolerincludes a current sensing circuit which is adapted to control thecurrent through the thermo-electric cooler using proportional, integral,derivative (PID) processing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention is best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion.

[0016]FIG. 1 is a schematic diagram showing a conventional voltage-modecontrol circuit for a TEC.

[0017]FIG. 2 is a schematic diagram of a current-mode TEC controlcircuit in accordance with an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION

[0018] In the following detailed description, for purposes ofexplanation and not limitation, exemplary embodiments disclosingspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone having ordinary skill in the art having had the benefit of thepresent disclosure, that the present invention may be practiced in otherembodiments that depart from the specific details disclosed herein.Moreover, descriptions of well-known devices, methods and materials maybe omitted so as to not obscure the description of the presentinvention.

[0019] Briefly, the present invention relates to a method and apparatusfor controlling the current through a thermal electric cooler duringoperation using a current-mode control circuit configuration.Advantageously, the TEC current is a substantially linear function ofthe TEC heat transfer. As will become more clear as the descriptionproceeds, the current-mode control in accordance with an exemplaryembodiment of the present invention enables TEC current control, theflow of which is directly related to the temperature differential acrossthe TEC, given a fixed thermal load. To this end, the thermal energy ofthe TEC is transferred by the majority carriers in the semiconductormaterial of the TEC, and is therefore substantially proportional to thecurrent, neglecting I²*R losses. Moreover, current-mode control of theTEC in accordance with an exemplary embodiment of the present inventionis substantially independent of the semiconductor resistance initialtolerance, to a first order approximation. This enables the variousperformance parameters of the current mode TEC control circuit of anexemplary embodiment of the present invention to be tightly controlled,and optimized. These and other advantages will become more apparent toone of ordinary skill in the art upon a review of the present inventionas described in connection with exemplary embodiments thereof.

[0020] As described, conventional voltage-mode control methods of athermo-electric cooler are inherently dynamically inaccurate because ofthe variation in voltage across the TEC from device to device as well asover-temperature. This inaccuracy translates into inaccuracies in theclosed-loop performance parameters (e.g., the transient response) of theTEC, and the attendant shortcomings of conventional voltage-mode TECcontrollers described in more detail above. In clear contrast, thecurrent-mode TEC controller in accordance with an exemplary embodimentof the present invention accurately controls the current through thethermoelectric cooler. This accuracy results from the proportionalrelationship between change in temperature across a thermo-electriccooler and the change in the current through the device.

[0021] Turning to FIG. 2, a current-mode TEC controller 200 according toan exemplary embodiment of the present invention is shown. Thecurrent-mode TEC controller 200 includes a control block 201. Thecontrol block 201 further includes a temperature monitor circuit 202which provides feedback to a PID controller 204. The PID controller 204includes a second input for the set-point 203 of the TEC temperature. Aswill become more clear as the description of the present exemplaryembodiment proceeds, the PID controller 204 changes the current of a TEC209, as is needed to maintain the sensed temperature at a level equal tothe set-point temperature. The TEC 209 is illustratively aPeltier-effect TEC.

[0022] A reference voltage (V_(ref)) 205 provides the offset necessaryfor bipolar control of the TEC current (to enable both heating andcooling) by the adjustment of the unipolar output of the PID controller204. The output from the PID controller 204 is a voltage between 0 andV_(ref) volts, which is calculated by the PID function to produce acurrent through the TEC necessary to achieve a TEC temperature equal tothe set-point input. The difference amplifier, comprising of the opamp207 and resistors R (208), R (208), R_(ref) (211) and R_(cntl) (212),controls the amount of current through the TEC.

[0023] When the value of R_(ref) (211) is set equal to the quantity(2*R_(cntl)+R), the TEC current (sensed by R_(sns)) will vary from(−R*V_(ref))/(R_(ref)*R_(sns)) to (+R*V_(ref))/(R_(ref)*R_(sns)) as thevoltage output of the PID control block is varied from 0 Volts toV_(ref), where V_(ref) can be any standard value reference, typicallyprovided for use by the DAC output of the PID block. The resistors R,R_(ref) and R_(cntl) are selected such that the TEC current is boundedwithin the TEC manufacturer's specified limits by the output range ofthe PID block (V_(ref)). This inherently provides the current limitingfunction. The output from the op amp 207 drives the high-current buffer210 such that the current that is sensed, or converted to a voltage, byR_(sns) 206, equals that determined by the PID control function. It isnoted that the current-mode controller of the exemplary embodiments ofthe present invention may be applied to both linear power as well asswitched-mode power designs. The current buffer block 210 shown in FIG.2 is a generic block that may utilize either method, depending upon therequirements of the application for efficiency, noise, etc.

[0024] The control block 201 is one portion of the closed-loop feedbackof the present invention. The input to the temperature monitor circuit202 is typically provided by a NTC thermistor 213. The temperaturemonitor circuit 202 provides an input voltage signal to the PID controlblock 204 that is representative of the temperature of the TEC 209. ThePID control block 204 also has a set-point 203 which is the targetvoltage level [representing the target TEC temperature] for operation ofthe TEC 209.

[0025] The PID control block 204 calculates the required change incurrent through the TEC 209 to effect the desired temperature change ofthe TEC to heat or cool the laser (not shown). The calculation isillustratively carried out using a closed-loop temperature controlmethod. An exemplary of closed-loop temperature control methodincorporates a combination of Proportional, Integral, and Derivative(PID) feedback in an attempt to achieve the optimal compromise betweenresponse time, accuracy, and stability. In a temperature controlapplication, the PID equation can be represented as: $\begin{matrix}{W = {{P \cdot \left( {T_{s} - T_{0}} \right)} + {I{\int{\left( {T_{s} - T_{0}} \right){t}}}} + {D \cdot \left\lbrack {\frac{}{t}\left( {T_{s} - T_{0}} \right)} \right\rbrack}}} & (1)\end{matrix}$

[0026] where T_(s) is the temperature setpoint or target, T₀ is thecurrent sensed temperature, W is the calculated power necessary to beapplied to the system in order to minimize the difference (T_(s)−T₀),which optimally would approach zero. The factors P, I and D areProportional, Integral, and Derivative constants, respectively. Thecombination of values of these parameters dictates the overall systemresponse, accuracy and stability. Many implementations of this methodset I and D both to zero and merely utilize proportional control becauseof the simplicity of stabilizing the closed-loop system.

[0027] The output from the PID controller 204 is the calculated controlvoltage which will provide the desired TEC current needed to effect thedesired temperature change of the TEC 209. Quantitatively, the TECcurrent may be expressed as (assuming a large open-loop OpAmp gain and aunity gain current buffer 210): $\begin{matrix}{I_{TEC} = \frac{\begin{matrix}\left\lbrack {{\left( {{R \cdot R_{ref}} + R^{2}} \right) \cdot V_{cntl}} + {\left( {{{- R} \cdot R_{ref}} - R^{2} - {R_{cntl} \cdot R_{ref}} - {R_{cntl} \cdot R}} \right) \cdot}} \right. \\\left. {V_{ios} + {\left( {{- R^{2}} - {R_{cntl} \cdot R}} \right) \cdot V_{ref}}} \right\rbrack\end{matrix}}{\left( {{R \cdot R_{sns} \cdot R_{ref}} + {R_{cntl} \cdot R_{sns} \cdot R_{ref}}} \right)}} & (2)\end{matrix}$

[0028] where V_(ios) is the input-offset voltage of the OpAmp 207; andV_(cntl) is the output voltage of the PID portion of the control block201.

[0029] In the event that symmetric heating and cooling is desired,R_(ref) (shown at 211 in FIG. 2) can be shown to be:

R _(ref)=2·R _(cntl) +R  (3)

[0030] where R_(cntl) is the control resistance 212 and R is theresistance of monitor resistors 208 shown in FIG. 2.

[0031] Moreover, when V_(ios)=0 in eqn. (2), and R_(ref) as given byeqn. (3) is substituted into eqn. (2), I_(TEC) becomes: $\begin{matrix}{I_{TEC} = {\left( {{2 \cdot V_{cntl}} - V_{ref}} \right) \cdot \frac{R}{R_{ref} \cdot R_{sns}}}} & \text{(3a)}\end{matrix}$

[0032] From eqn. (3a), it can be seen that the TEC current limit isbounded substantially by the range of the output voltage V_(cntl) ofcontrol block PID 204, which is from 0 volts to V_(ref).

[0033] To this end, from eqn. (3a), at the maximum V_(cntl) outputvoltage (V_(ref)), the maximum thermo-electric cooler current is boundedby: $\begin{matrix}{I_{{TEC\_}\max} = {\frac{R}{R_{ref}} \cdot \frac{V_{ref}}{R_{sns}}}} & (4)\end{matrix}$

[0034] One particularly useful aspect of the present exemplaryembodiment is the accuracy of the control of I_(TEC). To wit, usingstandard 1% resistors, the TEC current may be controlled toapproximately 2% or better. This allows the TEC 209 to be used forcooling up to within a couple percent of its maximum capacity, withoutentering into the self-heating range, thereby substantially preventingthermal runaway. Of course, this provides clear benefits to both theperformance and the life of the laser device being cooled by the TEC209.

[0035] From equations (2)-(4), it can be appreciated that the control ofthe TEC cooling is independent of the resistivity of the TEC device, toa first-order approximation, and independent of the associated voltagenon-linearities over temperature. Again, this independence of thevoltage across the TEC fosters accurate linear control of the TECtemperature independently of the intrinsic inaccuracies associated withvoltage-mode control. Also, the tolerance of all parameters may beselected to allow the precision of the design to be dictated by theapplication.

[0036] A more exact expression, which takes into account the finiteop-amp open-loop gain (A_(v)) the current buffer voltage-gain(A_(buffer)), as well as other second-order effects may be obtained bysumming currents at each current node in FIG. 2 and simultaneouslysolving the resultant equations. From this analysis, the current throughthe TEC may be expressed as: $\begin{matrix}{{I_{TEC}\left( V_{cntl} \right)} = {{- A_{buffer}} \cdot A_{v} \cdot \frac{\left( {{V_{ref} \cdot R} + {V_{ios} \cdot R_{ref}} - {2 \cdot V_{cntl} \cdot R} + {V_{ios} \cdot R}} \right)}{\begin{matrix}\left( {{R_{sns} \cdot R_{ref}} + {R_{TEC} \cdot R_{ref}} + {A_{v} \cdot A_{buffer} \cdot}} \right. \\\left. {{R_{sns} \cdot R_{ref}} + {R \cdot R_{sns}} + {R_{TEC} \cdot R}} \right)\end{matrix}}}} & (5)\end{matrix}$

[0037] where R_(TEC) is the intrinsic resistance of the TEC 209.

[0038] Certain aspects and advantages are worthy of specific mention atthis point. For example, because the current-mode TEC controller of theexemplary embodiment of the present invention directly senses thecurrent through the TEC via the sense resistor 206, and since thevoltage across this resistor is not needed for current limiting as inconventional voltage-mode control techniques, the current sense resistor(R_(sns)) 206 may be made arbitrarily small The value of resistor 206 issubstantially limited only by the input offset voltage of OpAmp 207(e.g.: 0.1 ohms will result in an approximate error of 10 mA for an opamp with an input offset voltage (V_(ios)) of 1 mV) and the tolerance ofmonitoring resistors 208 as can be shown by partial differentiation ofeqn. (2) (or even more precisely eqn. (5), depending on the desiredaccuracy). This ultimately enables the sense resistor 206 to bephysically much smaller than sense resistors used in conventionalvoltage-mode control circuits, for example that shown in FIG. 1.

[0039] This reduction in the size of the sense resistor 206 helpsconserve valuable board space in a deployed TEC device. Moreover,because the resistance value of the sense resistor 206 is relatively lowwhen compared to conventional voltage-mode sense resistors, a lowersupply voltage may be used to power the TEC. In contrast, the senseresistor of a conventional voltage-mode control circuit can consume onthe order of approximately 20% of the supply voltage, thereby limitingthe heating/cooling range of the TEC by limiting the available supplyvoltage. Furthermore, because of the relatively low resistance of senseresistor 206, a relatively insignificant amount of heat is dissipated bythis device at maximum TEC current. Of course, this reduces thecontribution of the sense resistor 206 to thermal run-away, and is,therefore, another significant advantage when compared to theconventional voltage-mode TEC control circuit.

[0040] Finally, it is noted that limiting the TEC current in accordancewith the present exemplary embodiment does not require transistor-basedsensing circuitry. Among other advantages, this avoids the limiting ofthe TEC operational range as may occur in conventional voltage-mode TECcontrollers as a result of the temperature dependent V_(be) associatedwith Q_(p) and Q_(n) in FIG. 1.

[0041] The invention having been described in detail in connectionthrough a discussion of exemplary embodiments, it is clear that variousmodifications of the invention will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure. Suchmodifications and variations are included within the scope of theappended claims.

I claim:
 1. An apparatus for controlling the operating temperature of athermo-electric cooler, comprising: a current-sense circuit which isadapted to control a current through the thermo-electric cooler based ona set-point temperature and a detected temperature of thethermo-electric cooler using proportional, integral, derivative (PID)processing.
 2. An apparatus as recited in claim 1, wherein saidcurrent-sense circuit limits a maximum current through thethermo-electric cooler.
 3. An apparatus as recited in claim 1, whereinsaid current through the TEC is controlled to approximately 2% orbetter.
 4. An apparatus as recited in claim 1, further comprising acontrol block.
 5. An apparatus as recited in claim 4, wherein thecontrol block further comprises a temperature monitor circuit and aproportional, integral, derivative (PID) controller.
 6. An apparatus asrecited in claim 1, further comprising a difference amplifier circuit.7. An apparatus as recited in claim 1, wherein a sense resistor isoperatively connected to said thermo-electric cooler.
 8. An apparatus asrecited in claim 6, wherein said difference amplifier further comprisesan operational amplifier and a plurality of resistors including a senseresistor.
 9. An apparatus as recited in claim 8, wherein said differenceamplifier controls an amount of current through the thermo-electriccooler.
 10. An apparatus as recited in claim 8, wherein said pluralityof resistors are selected such that a maximum current through thethermoelectric cooler is not exceeded.
 11. An apparatus for controllingthe operating temperature of a thermo-electric cooler, comprising: acurrent-sense circuit which is adapted to control a current through thethermo-electric cooler.
 12. An apparatus as recited in claim 11, whereinsaid current-sense circuit limits a maximum current through thethermo-electric cooler.
 13. An apparatus as recited in claim 11, whereinsaid current through the TEC is controlled to approximately 2% orbetter.
 14. An apparatus as recited in claim 11, further comprising acontrol block.
 15. An apparatus as recited in claim 14, wherein thecontrol block further comprises a temperature monitor circuit and aproportional, integral, derivative (PID) controller.
 16. An apparatus asrecited in claim 11, further comprising a difference amplifier circuit.17. An apparatus as recited in claim 11, wherein a sense resistor isoperatively connected to said thermo-electric cooler.
 18. An apparatusas recited in claim 16, wherein said difference amplifier furthercomprises an operational amplifier and a plurality of resistorsincluding a sense resistor.
 19. An apparatus as recited in claim 18,wherein said difference amplifier controls an amount of current throughthe thermo-electric cooler.
 20. An apparatus as recited in claim 18,wherein said plurality of resistors are selected such that a maximumcurrent through the thermo-electric cooler is not exceeded.